The present invention relates to a motor controller and related method. More particularly, but not exclusively the invention relates to a motor controller, specifically, though not exclusively, for use with an alternating current (AC) induction motor.
AC motors, that typically run off a 50 Hertz (Hz) AC supply, are cheap to manufacture and reliable to operate. AC motors are found in a myriad of industrial applications, including: pumps, compressors, fans and drive systems. AC motors can be sub-divided into types according to the number of phases of the power supply. For example, AC motors may be single or three phase. Three phase AC motors tend to be more efficient than single phase motors because of the way single phase motors are usually connected to a domestic single phase power supply. It is usual for one winding to be directly connected to the supply and the other winding via a capacitor. This is known as a single phase capacitor run motor. Such motors are used extensively for power ratings between a few tens of Watts to several kilowatts. However for powers above this range, and as a result of their inherent greater efficiency and smoothness, three phase induction motors have been adopted as the workhorse of many drive systems.
In recent years there has been growing legislative pressure to improve inefficient machines, reduce unnecessary energy waste and minimise so-called ‘carbon related emissions’ from the perspective of their effect on environmental conditions. Pressure is in the form of lobbying, private pressure groups and also from increasingly stringent legislation and is particularly directed at electric motors.
Designers of electric motors and motor controllers have therefore been focusing on ways of improving the efficiency of motors and their controllability, with a view to operating them at optimum conditions so as to extract more power. The single phase capacitor run induction motor in particular has been singled out as not being very efficient. As a solution to this, considerable effort has resulted in the design of electrically commutated motors (ECM) or brushless DC motors. These employ a permanent magnet rotor which is marginally more efficient than a rotor whose magnetic field is derived from a circulating electrical current and the consequent resistive losses in such a rotor. However, ECMs are more expensive and the magnet is prone to degrade with time.
Another problem with ECMs is the fact that they require a controller to be incorporated into the motor housing and the controller can suffer from the effects of heat, vibration and moisture ingress.
The efficiency of the ECM is quite high but they can create problems, as their apparent high electrical efficiency belies a significant additional load to the utility and unless extra components are added (usually externally) they exhibit a poor power factor and high peak current crest value, as well as giving rise to high inrush currents on power up.
It should also be noted that the production and disposal of permanent magnets gives rise to potentially environmentally damaging pollutants that require specialised handling and treatment. This raises issues for both their manufacture and end of life recycling.
International Patent Application WO 03/084047 (Mol Belting Company) discloses a controller for a brushless motor having a power factor correction element which ensures near sinusoidal current consumption from a low frequency supply main (50 Hz or 60 Hz) and also acts as a fixed voltage regulator for delivering regulated voltage to a drive. While such additional elements are desirable, they fail to offer in-rush current protection and fail to address high voltage switching transients on the motor windings at low load levels.
One area of particular interest is AC motors that are rated at 240 volts but driven with an input voltage of around 50-180 Volts. A reason for this is that these motors are very common as they are used in so many domestic and industrial systems such as: coolers, ventilation and air conditioners and as such manufacturers are keen to see them meet the increasingly stringent environmental demands.
The aforementioned AC induction motors have however suffered from a number of drawbacks. The use of the capacitor to obtain the current-phase lead in one winding is not very efficient and also when either triac/silicon controlled rectifier (SCR) phase controllers or auto transformers are used to reduce speed and/or torque, additional losses are introduced as a result of operating at non-optimum condition, for example because of increased slip.
The losses and inefficiencies were in fact due to the imbalance that often exists between the requirements of a mechanical load, the way a motor operates and the control and conditioning of the available power. The net result is that a significant amount of energy is wasted.
Thus the present invention is principally concerned with improving the efficiency of induction motors, particularly AC induction motors, by providing a more suitable method of controlling and conditioning electrical input current and voltage, taking into account the way the induction motor works and the requirements of the mechanical load.